The advent of two technologies has enabled a streamlined method of producing complete prescription eyeglass lenses called on-block manufacturing (OBM). The first technology is a plastic and ultraviolet (UV) cured adhesive that allows lens blanks to be coupled with a lens blocking piece, also known as a block, a blocker, a lens chuck, or a surface block, that can hold lens blanks for machining. The second technology is the Full Back Side (FBS) digital surfacing technology, also referred to as free form technology or free form generation, which allows the entire prescription to be cut into the back (usually concave) surface of a lens. By convention, the back or rear surface of the lens is the surface closest to the wearer's eyes, and the front surface of the lens is the surface opposite the back surface. For a typical convex-concave lens, the front surface is convex and the back surface is concave. Other lenses may be biconcave or biconvex, with front and back surfaces that are both concave or both convex, respectively.
FIG. 1 illustrates a conventional OBM process. In this OBM process 100, based on a prescription, an appropriate lens blank with a fully processed front (usually convex) surface is first selected such that the OBM process only treats the back surface to save manufacturing time and streamline the manufacturing process. Then the lens blank is coupled to a lens block, in a step called blocking, by gluing the front surface of the lens blank to the lens block using a UV-curable adhesive. The lens block holds the lens blank while the concave surface of the lens blank is machined to fill the prescription. To further reduce manufacturing time, lens blanks can be pre-blocked, i.e., the lens blank can be affixed, bonded, glued, or otherwise secured to the lens block before arriving at the OBM facility.
After the lens blank has been blocked (also referred to as lens on block), its back surface is machined in two phases: a coarse machining phase at step 110 to generate the overall shape (e.g., using a generator), and a fine machining phase at step 130 to polish the surface and achieve the desired surface qualities. An engraving step 120 can be performed between the coarse machining and the fining machining to engrave semi-visible and/or visible marks on the lens to, for example, guide subsequent manufacturing steps. After the back surface machining, the back surface is usually cleaned at step 140 and dried at step 150 before being coated with, for example, a hard coating at step 160 and/or an anti-reflection coating at step 170. Then the coated lens is removed, in a step 180 called deblocking, from the lens block for edging, which involves cutting the lens into an appropriate shape to fit the lens frame. An off-block inspection step 190 can be performed after the lens is removed from the block.
The OBM process can normally produce a pair of eyeglass lenses in less than a business day or two. Depending on the business model, some OBM labs offer a guaranteed delivery time of less than 8 hours, less than 3 hours, or less than 90 minutes. The guaranteed delivery time can be measured from receiving a prescription to a point at which the framed eyeglasses are ready for shipment. In some fast OBM labs, for example in urban areas, the guaranteed delivery time can include the shipment as well.
Unfortunately, defects near (on or beneath) the back surface of the lens blank may render the finished lens unsuitable for filling the prescription, i.e. the lens parameters are out of a specified range of tolerance. These defects include but are not limited to scratches, dirt, cracks, smudges, or pieces of lint on the lens surface. Defects may be introduced during the machining process or from exposure to the surrounding environment during the OBM process. A hairline scratch on the lens surface introduced during the fine machining phase, for example, can cause a defective anti-reflection coating, reducing the manufacturing yield.
To increase manufacturing yield, a lens may be inspected for defects before coating. However, during conventional inspection, an inspector looks through the lens to identify any defect. Accordingly, the lens is usually deblocked before inspection in order to transmit light through the front and back surfaces, and reblocked again after inspection for subsequent processing steps (e.g., coating). Deblocking and reblocking consumes extra time and can disrupt the OBM process.